Since the Nixon years, the GOP has enjoyed a marked advantage over the Democrats at the presidential level. Cultural issues -- race, religion, abortion, patriotism -- have worked to the Republicans' advantage. Until Barack Obama's election in 2008, no Democrat has won the presidency without aggressively adapting to that fact.
Jimmy Carter, a Southern evangelical, defeated a very moderate Vice President Gerald Ford at a moment when the GOP was still reeling from the Watergate scandal and Ford's pardon of Nixon. Still, it's easy to forget that Carter was the choice of what would come to be known as the "Christian Right" (historian Steven Hayward reminds me that Carter got a lavishly generous reception on the fledgling "700 Club").
At the beginning of the general election campaign, Ford was behind 33 points and by the end had very nearly caught up with Carter. Had Carter not cast himself as a pious Southern veteran, small business owner, and conservative-leaning Democrat (opposed to busing, muddled on abortion), or if the GOP had not nominated such a liberal candidate so closely tied to the Nixon presidency, it's quite possible the Republicans would have won in 1976.
Since then, any Democrat who tried to run as an unapologetic liberal, particularly on cultural issues, lost. In 1992, the Democrats seemed to have figured that out. They nominated Bill Clinton, a pro-death penalty Southerner who wanted abortion to be "safe, legal and rare" and who campaigned against the "something for nothing" welfare state politics of his own party. He picked fights -- often symbolic, occasionally substantive -- with the left to prove he was a centrist.
Thanks to a host of complex reasons, many of them having to do with George W. Bush's mixed political legacy, the Democrats no longer feel the need to play Clintonian games. They yanked the "rare" language out of the abortion plank of the platform, supporting instead abortion on demand. Indeed, a layman watching the speeches from abortion activists, and more importantly the reactions of the delegates, would conclude that the ancestral party of the Free Silver movement had morphed into the free abortion movement. It's a remarkable shift given that the electorate is more pro-life today than it was when Bill Clinton ran in 1992.